Subset function to check if one collection is a subset of another (#89)

* Create subset.go

* Update subset.go

* Update subset.go

* Update subset.go

* Create subset_test.go

* Update subset.go

* Update subset_test.go

* Update subset_test.go

subset.go

@@ -0,0 +1,43 @@
+package funk
+
+import (
+	"reflect"
+)
+
+// Subset returns true if collection x is a subset of y.
+func Subset(x interface{}, y interface{}) bool {
+	if !IsCollection(x) {
+		panic("First parameter must be a collection")
+	}
+	if !IsCollection(y) {
+		panic("Second parameter must be a collection")
+	}
+
+	xValue := reflect.ValueOf(x)
+	xType := xValue.Type()
+
+	yValue := reflect.ValueOf(y)
+	yType := yValue.Type()
+
+	if NotEqual(xType, yType) {
+		panic("Parameters must have the same type")
+	}
+
+	if xValue.Len() == 0 {
+		return true
+	}
+
+	if yValue.Len() == 0 || yValue.Len() < xValue.Len() {
+		return false
+	}
+
+	for i := 0; i < xValue.Len(); i++ {
+		if !Contains(yValue.Interface(), xValue.Index(i).Interface()) {
+			return false
+		}
+		return true
+	}
+
+	return false
+
+}

subset_test.go

@@ -0,0 +1,31 @@
+package funk
+
+import (
+	"testing"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestSubset(t *testing.T) {
+	is := assert.New(t)
+
+	r := Subset([]int{1, 2, 4}, []int{1, 2, 3, 4, 5})
+	is.True(r)
+
+	r = Subset([]string{"foo", "bar"},[]string{"foo", "bar", "hello", "bar", "hi"})
+	is.True(r)
+
+	r = Subset([]string{"hello", "foo", "bar", "hello", "bar", "hi"}, []string{})
+	is.False(r)
+
+        r = Subset([]string{}, []string{"hello", "foo", "bar", "hello", "bar", "hi"})
+	is.True(r)
+
+	r = Subset([]string{}, []string{})
+	is.True(r)
+
+	r = Subset([]string{}, []string{"hello"})
+	is.True(r)
+
+	r = Subset([]string{"hello", "foo", "bar", "hello", "bar", "hi"}, []string{"foo", "bar"} )
+	is.False(r)
+}